I spoke to the mechanic involved in the e15 head a12 speedway outfit that rallycar1200 got me onto and he can do the a15 with the e15 head with modded almost de-counterway crank and modded journals to accept hayabusa rods, his specially balanced alloy flywheel to suit the lightweight (not knife edged) crank and rods and some off the shelf 80s motorbike forged pistons to suit the 2v ohc chamber head. He predicts 200+ engine hp from his a12 engine but a bigger 380cc and longer stroke crank could maybe be similar with more torque. Also he uses flatslides.
This of course is a high revving engine and maybe not as streetable as desired.
He can also do gearbox adapters for a series to toyota and nissan L series gearboxes.

The E- series & A- series engines use the same piston design. matter of fact, I have used pistons from an A-14 that had a smaller cc crown in an E-16. The stock bore size of an E- sereis engine is 2.992. You can order stock A-14 pistons with smaller crown 0.050 over & will equal 2.992, & I had the head cut 0.050. I have a head & lbock from both. I'll set it up and see what is goin on .

The E- series & A- series engines use the same piston design. matter of fact, I have used pistons from an A-14 that had a smaller cc crown in an E-16. The stock bore size of an E- sereis engine is 2.992. You can order stock A-14 pistons with smaller crown 0.050 over & will equal 2.992, & I had the head cut 0.050. I have a head & lbock from both. I'll set it up and see what is goin on .